Deep catalytic cracking process

ABSTRACT

The present invention provides a catalytic cracking reactor system and process in which a riser reactor is configured to have two sections of different radii in order to produced improved selectivity to propene and butenes as products.

FIELD OF THE INVENTION

[0001] The present invention relates to a process for convertinghydrocarbon feedstocks to cracked products. More particularly, thepresent invention relates to an improved process for producing olefinsfrom hydrocarbon feedstocks. Most particularly, the present inventionrelates to an improvement to deep catalytic cracking processes.

BACKGROUND OF THE INVENTION

[0002] The processes of non-catalytically cracking and catalyticallycracking hydrocarbon feedstocks are well known in the art. In thisregard, steam cracking in a furnace and contact with hot non-catalyticparticulate solids are two well-known examples of non-catalytic crackingprocess. Examples of non-catalytic cracking processes of these types aredisclosed in, for example, Hallee et al., U.S. Pat. No. 3,407,789;Woebcke, U.S. Pat. No. 3,820,955; DiNicolantonio, U.S. Pat. No.4,499,055 and Gartside et al., U.S. Pat. No. 4,814,067.

[0003] Additionally, catalytic cracking processes are known. Forexample, fluid catalytic cracking processes have been described in thepatent literature in Cormier, Jr. et al., U.S. Pat. No. 4,828,679; Raboet al., U.S. Pat. No 3,647,682; Rosinski et al., U.S. Pat. No.3,758,403; Lomas, U.S. Pat. No. 6,010,618; and Gartside et al., U.S.Pat. No. 5,324,484. Special mention is made of the improvements in fluidcatalytic cracking described in Letzsch et al., U.S. Pat. No. 5,662,868and Letzsch et al., U.S. Pat. No. 5,723,040.

[0004] Another catalytic cracking process that is especially useful inproducing olefins from hydrocarbon feedstocks has been termed the deepcatalytic cracking processes. The deep catalytic cracking process isdescribed in Li et al., U.S. Pat. No. 4,980,053; Yongqing et al., U.S.Pat. No. 5,326,465; Shu et al., U.S. Pat. No. 5,232,675; Zhicheng etal., U.S. Pat. No. 5,380,690 and Zaiting et al., U.S. Pat. No. 5,670,037.

[0005] The olefins produced in these processes have long been desired asfeedstocks for the petrochemical industries. Olefins such as ethylene,propylene, the butenes and the pentenes are useful in preparing a widevariety of end products, including but not limited to polyethylenes,polypropylenes, polyisobutylene and other polymers, alcohols, vinylchloride monomer, acrylonitrile, methyl tertiary butyl ether and otherpetrochemicals, and a variety of rubbers such as butyl rubber.Additionally, the heavier hydrocarbons produced in the processes havealso been long desired for use as gasolines, fuels and light cycle oils.

[0006] It would constitute a significant advancement in the state of theart if an improved deep catalytic cracking process could be developedwhich could handle a wide variety of feedstocks and crack thesefeedstocks to more valuable olefins (C₃ and C₄ olefins) with less C₂olefins. It would represent a further significant advancement in thestate of the art if the improved process could be readily and easilyapplied to revamping an existing catalytic cracking process.

SUMMARY OF THE INVENTION

[0007] To this end, the present inventor has now discovered an uniqueimproved deep catalytic cracking process comprising a first crackingreaction operated at relatively high weight hourly space velocity toconvert approximately 35 to 60 percent of the feed and a second crackingoperation operated at relatively low weight hourly space velocity tocomplete conversion, that overcomes the drawbacks of the prior art andmeets the commercially desirable aspects discussed above.

[0008] In view of the foregoing, it is an object of the presentinvention to provide an improved deep catalytic cracking process.

[0009] It is a further object of the present invention to provide animproved deep catalytic cracking process that produces increased yieldsof C₃ and C₄ olefins at the expense of C₂ olefins.

[0010] It is another object of the present invention to provide animproved deep catalytic cracking process that can be employed in anexisting cracking facility that can easily be revamped to accommodatethe improved process of the present invention.

[0011] These and other objects evident to those of ordinary skill in theart are provided by the present invention discussed in detailhereinbelow.

BRIEF DESCRIPTION OF THE DRAWING

[0012] The FIGURE depicts a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention provides an improved process for deepcatalytic cracking of a hydrocarbon feedstock. In conventional deepcatalytic cracking process, a preheated hydrocarbon feedstock is crackedover heated solid acidic catalyst in a reactor at temperatures rangingfrom about 925° F. to about 1350° F., preferably from about 1025° F. toabout 1150° F. The weight hourly space velocity of the feedstock chargemay range from about 0.2 hr⁻¹ to about 20 hr⁻¹. The catalyst-to-oilratio may vary from about 2 to about 12, preferably from about 5 toabout 10. In order to lower the partial pressure of the hydrocarbonfeed, steam or other gases, such as the dry gas of a catalytic crackingunit, may be added into the reactor during the conversion process.

[0014] When steam is used, a weight ratio of steam to hydrocarbon feedis preferably maintained at from about 0.01 to 0.5:1. The total pressureof the reaction preferably ranges from about 20 psia to about 45 psia,more preferably from about 25 psia to about 35 psia.

[0015] After the reaction, the spent catalyst particles may be steamstripped to remove residual hydrocarbons absorbed on or entrained withthe catalyst as is known in the art. The spent catalyst particles withcoke deposited thereon are then transferred to a regeneration zone as isalso well known to those of ordinary skill in the art.

[0016] Regeneration of the catalyst is then generally conducted bycontacting the catalyst with an oxygen-containing gas at a temperatureranging from about 1175° F. to about 1350° F. Afterwards the regeneratedcatalyst is recycled to the reaction zone.

[0017] Hydrocarbon feedstocks useful in the DCC processes of the priorart, and in accordance with this invention, may vary in a wide range,and typically are relatively heavy hydrocarbon feedstocks, such as thoseselected from petroleum fractions with different boiling ranges, i.e.,naphtha, gas oil, vacuum gas oil, residual oil, crude oil and mixturesthereof.

[0018] Catalysts for use in the DCC processes of the prior art and inaccordance with the present invention are solid, acidic catalystscomprising one or more active components and a matrix material. Theactive components include amorphous aluminosilicates or zeolites such aspentasil shape selective molecular sieves, faujasite, ZSM-5 typecatalysts, rare earth cation exchanged faujasite, chemically treatedand/or stabilized faujasite and mixtures thereof. The matrix materialincludes synthetic inorganic oxides and mineral clays. All of thesecatalysts are available commercially.

[0019] Exemplary of the useful catalysts are pentasil shape molecularsieves, ZSM-5, rare earth exchanged Y sieve (REY) containing catalysts,pentasil shape molecular sieves supported on kaolinite, amorphousaluminosilicates and mixtures of any of the foregoing. For a moredetailed description of the conventional DCC process and the usefulcatalysts and variations on the DCC process reference is made to Li etal., U.S. Pat. No. 4,980,053; Yongqing et al., U.S. Pat. No. 5,326,465;Shu et al., U.S. Pat. No. 5,232,675; Zhicheng et al., U.S. Pat. No.5,380,690; Yukang et al., U.S. Pat. No. 5,358,918 and Zaiting et al.,U.S. Pat. No. 5,670,037. Reference is also made to Argauer et al., U.S.Pat. No. 3,702,886 for a description of ZSM-5.

[0020] As mentioned above, the present invention provides an improvedprocess over the DCC processes of the prior art, by providing for thecracking in two separate and distinct cracking zones, the firstoperating at a relatively high weight hourly space velocity, and thesecond operating at a relatively low weight hour space velocity. TheFIGURE shows a schematic arrangement of a riser and regenerator systemuseful in the practice of the present invention. The arrangement of theFIGURE comprises a first narrow riser section 2, a broader riser section4, a disengager vessel 6, a stripper 8 and a regenerator 10 thatprovides for a conversion zone and for the pneumatic conveyance ofcatalyst. The arrangement circulates the catalyst and contacts the feedin the manner described hereinbelow.

[0021] Regenerated catalyst from catalyst regenerator 10 passesdownwardly from the regenerator 10 through a line 12 to the bottomportion of the first narrow riser section 2. Appropriate control valvemeans (not shown) can control the flow of catalyst into and through theline 12. A lift fluid (optional) enters the bottom of the first narrowriser section 2 through a line 14 and transports the regeneratedcatalyst up through the riser 2 into contact with a feedstream. Feedinjection nozzles 16 inject the feedstream into the flowing regeneratedcatalyst and the mixture of feed and catalyst continue upwardly throughthe riser.

[0022] The first narrow riser section 2 will have a length and radiussufficient to provide a weight hourly space velocity in the first narrowriser section 2 ranging from about 50 to about 200 hr⁻¹ or more,preferably in the range of from about 70 to about 80 hr⁻¹. In this firstnarrow riser section 2, the conversion of the feed is from about 35 toabout 60%, and the cracking conditions are set so that the conversion isselective to the creation of gasoline from the feedstock. For example,the temperature in the first narrow reaction section preferably rangesfrom about 925° F. to about 1350° F., preferably from about 1000° F. toabout 1150° F. In order to lower the partial pressure of the hydrocarbonfeed, steam or other gases, such as the dry gas of a catalytic crackingunit, in amounts up to about 20 weight percent based on the weight ofthe feedstock, may be added into the first narrow riser section 2 via aconduit 18, which can be located either upstream or downstream of thefeed injectors.

[0023] The first narrow riser section 2 then proceeds through a diametertransition zone 20 and into the second broader riser section 4. Theratio of the radius of the second broader riser section 4 to the radiusof the first narrow riser 2 section should range from about 1.1:1 toabout 5.0:1, preferably from about 1.25:1 to about 2.5:1, or from about1.5:1 to about 2.5:1. Typically, the radius of the first narrow risersection 2 can range from about six inches (6″) to about eight feet (8′),preferably from about two feet (2′) to about six feet (6′), and theradius of the second wider riser section 4 can range from about nineinches (9″) to about sixteen feet (16′), preferably from about threefeet (3′) to about ten feet (10′). However, the actual radius size willgenerally be dependent upon the amount of hydrocarbon feedstock which isbeing cracked in the reactor. Also preferred is where the diametertransition zone 20 effects a relatively smooth diameter transition, suchas at an angle 22 ranging from about 5° to about 30°, preferably fromabout 8° to about 20°.

[0024] In the second broader riser section the weight hourly spacevelocity is significantly slowed, such as to a weight hourly spacevelocity of less than about 100 hr⁻¹, such as from about 90 hr⁻¹ toabout 2 hr⁻¹, more preferably from about 50 hr⁻¹ to about 8.0 hr⁻¹. Thetemperature in the second broader riser section will preferably rangefrom about 900° F. to about 1250° F., preferably from about 975° F. toabout 1050° F. At these conditions, the conversion of the feedstock iscompleted, and the gasoline produced in the first narrow riser section 2is selectively converted at high yields to LPG, i.e., C₃ and C₄ olefinswith significantly less production of ethylene than in prior art deepcatalytic conversion or fluid catalytic conversion processes. Inpreferred embodiments, dilution steam or other gases for lowering thepartial pressure of the hydrocarbons may be added in amounts up to about20 weight percent based on the weight of the feedstock to the secondbroader riser section 4 via a line 24.

[0025] The lengths of the first narrow riser section 2 and the secondbroader riser section 4 are set in order to effect the requisiteconversions in accordance with the operating conditions in the risersections, the type of feedstock and the type of catalyst employed, aswill be apparent to those skilled in the art. As a non-limiting example,the first riser section can have a length of about 40 feet and a thesecond riser section can have a length of about 25 feet where the ratioof the radius in the second riser section to the first riser section isabout 1.5:1.

[0026] After completion of the conversion in the second broader risersection 4, the riser narrows to termination section 26 that directs thecracked gases into a crossover duct 28 or other riser terminationdevices as are well known to those skilled in the art. The radius of thetermination section 26 is not critical to the present invention,however, preferably the radius of the termination section 26 isapproximately the same as that of the first riser section 2. The radiusof the termination section 26 should be sufficient to accommodate theincreased volume of lighter components and is suitable for attaching tothe crossover duct. Again, as with the first diameter transition zone22, a second transition zone 25 is preferred between the second broaderriser section 4 and termination section 26 which effects a relativelysmooth diameter transition, such as at an angle 27 ranging from about 5°to about 30°, preferably from about 8° to about 20°.

[0027] At the riser top 28, the cracked vapors are discharged into gasrecovery conduits 30 that direct the cracked gas vapors mixed with spentcatalyst into cyclones 32. Spent catalyst is separated from the crackedgas vapors in the cyclones 32 and the spent catalyst falls out ofcyclone 32 through dipleg 34 at a rate regulated by a dipleg sealingmeans, such as a flapper valve, 36. Secondary cyclones (not shown) mayalso be included in the disengaging vessel 6 to separate catalyst finesfrom the cracked vapors as is well known in the art.

[0028] The cracked gases are then removed from the cyclone 32 (and thenpreferably further separated from catalyst fines in secondary cyclones)and out of the disengaging vessel via a conduit 42 for furtherprocessing and recovery of the high yields of propene and butenesproduced in accordance with the present invention, along with otherproduct hydrocarbons. In preferred embodiments, where desired, quench,injected via lines 50; may be added to the cracked gases in conduit 42for quenching residual cracking reactions. The quench injection nozzles50 add a quench fluid that for quenching the cracking reactions, boththermal and catalytic, which may be continuing. Suitable quench fluidscan comprise kerosene, light coker gas oil, coke still (coker)distillates (CSD), hydrotreated distillate, light catalytic cycle oil(LCCO or LCO), heavy catalytic cycle oil (HCCO or HCO), heavy catalyticnaphtha (HCN), fresh unprocessed virgin feedstocks such as virgin gasoil, heavy virgin naphtha, light virgin naphtha, water, steam andmixture or a combinations of any of the foregoing. Desirably, the quenchshould have low thermal reactivity. Previously cracked hydrocarbons aregenerally desirable because they are less reactive to thermal quenchingthan fresh unprocessed virgin feedstocks and hydrotreated feedstocks.Quenching is more fully described in the published art, for example, inForgac et al., U.S. Pat. No. 5,043,058.

[0029] Disengaging vessel 6 serves as a containment vessel that housesthe upper portion of the riser reactor, the catalyst/cracked productvapor separation equipment and the stripper 8. The catalyst isdischarged into a lower portion of the dilute phase 38 of thedisengaging vessel 6 at a point above the surface 40 of the dense phasecatalyst bed 44. Disengaging vessel 6 also confines gases passing acrossthe surface of dense phase catalyst bed 44. Gases in the upper dilutephase 38 are recovered in cyclone separators (not shown) and anyseparated catalyst particles or fines are returned to the dense phasecatalyst bed 44.

[0030] The catalyst in dense bed 44 may then conveniently be stripped inthe stripper section 8 of the disengager vessel 6 by injection of steamor other stripping fluid (not shown). In preferred arrangements, baffles46 are provided in the stripping section 8 to aid in displacing anyproduct gases from the catalysts as the catalyst proceeds downwardlythrough the stripping section 8. Alternatively, packing can also be usedin the stripping stage as is known to those skilled in the art.

[0031] The stripped catalyst is then removed from the bottom of thedisengaging vessel via a line 48 for transport to the catalystregenerator 10. Any of a number of regenerator designs well known tothose skilled in the art may be employed in connection with the presentinvention. An exemplary two-stage regenerator design, as shown in theFIGURE, is described below, although, as indicated above, any type ofregenerator may be used in accordance with the present invention,including those with and without catalyst coolers.

[0032] The catalyst from line 48 is directed into a dense fluidizedcatalyst bed 52 in the lower stage 54 of a two-stage regenerator vessel10. In the dense fluidized catalyst bed 52, catalyst is contacted withan oxygen-containing regeneration gas introduced into the lower bed 52via a line 56 and distribution ring 57. Regeneration zone 54, asoperated in accordance with procedures known in the art, is maintainedunder conditions as a relatively low temperature regeneration operationgenerally below 1300° F. Conditions in the lower regeneration zone 54are selected to achieve at least partial combustion and removal ofcarbon deposits and substantially all of the hydrogen associated withthe deposited hydrocarbonaceous material from catalytic cracking.

[0033] The combustion accomplished in the lower regeneration zone 54 isthus accomplished under conditions to form a carbon monoxide rich firstregeneration zone flue gas in an upper dilute phase 58 of the lowerregeneration zone. The flue gas is separated from entrained catalystfines by one or more separators, such as cyclones 60 and 62. Catalystthus separated from the carbon monoxide gases by the cyclones 60 and 62,is returned to the catalyst bed 52 via diplegs 64 and 66, respectively.Carbon monoxide rich flue gases recovered from the cyclones 60 and 62 isdirected via conduit means 68 and 70, respectively, to a carbon monoxideboiler or incinerator and/or a flue gas cooler (both not shown) togenerate steam by a more complete combustion of available carbonmonoxide therein, prior to combination with other process flue gasstreams for venting or other processing.

[0034] In the first regeneration zone, it is therefore intended that theregeneration conditions are selected such that the catalyst is onlypartly regenerated by the removal of hydrocarbonaceous depositstherefrom, i.e., removal of from about 40 to about 80% of the cokedeposited thereon. Sufficient residual carbon is intended to remain onthe catalyst to achieve higher catalyst particle temperatures in asecond catalyst regeneration zone 72, i.e., above about 1300° F., asrequired to achieve virtually complete removal of the carbon from thecatalyst particles by combustion thereof with excess oxygen-containingregeneration gas.

[0035] Accordingly, the partially regenerated catalyst from the firstregeneration zone bed 52, now substantially free of hydrogen and havinglimited residual carbon deposits thereon, is withdrawn from a lowerportion of bed 52 for transfer upwardly through riser conduit 74 todischarge into the lower portion of a dense fluid bed of catalyst 76 inupper regeneration zone 72. Lift gas, such as compressed air, is chargedinto the bottom of riser conduit 74 via a line 78.

[0036] Conditions in the upper regeneration zone 72 are designed toaccomplish substantially complete removal of carbon from the catalystnot removed in the lower regeneration zone 54, as discussed above.Accordingly, regeneration gas such as air or oxygen-enriched gas ischarged to bed 76 by conduit means 80 and distributor ring 82.

[0037] Cyclones (not shown) may be present in the upper dilute phase 84of the upper regeneration zone 72 or they may be located externalthereto depending on the temperature in the upper zone and constrains ofthe materials of construction for the cyclones. In either event, theupper regeneration cyclones operate similar of those of the lower zone(cyclones 60 and 62) and separate the flue gas from the catalystparticles, withdrawing the flue gas via a line 86 and returning theseparated catalyst particle to the bed 76. The fully regeneratedcatalyst at the bottom of bed 82 is then removed from the upper stage 72of the regenerator vessel 10 via a standpipe 12 and directed to thelower portion of the riser reactor 2.

[0038] In the revamping mode, one skilled in the art can see in light ofthe above-description, that a typical fluid catalytic cracking system,or other cracking reacting system employing a riser-type reactor, canreadily be converted into the improved deep catalytic cracking system ofthe present invention. A portion of the existing riser in the FCCfacility can be cut and removed, and replaced by welding with a sectionof the broader reaction zone and transition zones. In this manner,conversion of FCC plants directed to increased gasoline production, canbe revamped into an improved DCC plant that produces high yields ofpropene and butenes.

[0039] Many variations of the present invention will suggest themselvesto those skilled in the art in light of the above-detailed description.For example, the quench injection may be added at the two pointssuggested in FIG. 1, may not be added at all, or may be added at anypoint downstream. Further, the cyclone separator may or may not be closecoupled to the riser terminator. Other types of gross cut separators maybe employed in addition to the cyclones, such as a ramshorn separator,an inverted can separator, or a globe separator. See, for example, theseparators shown in Pfeiffer et al., U.S. Pat. No. 4,756,886, Haddad etal., U.S. Pat. No. 4,404,095; Ross et al., U.S. Pat. No. 5,259,855,Barnes, U.S. Pat. No. 4,891,129 and/or Gartside et al., U.S. Pat. No.4,433,984. Other regenerator configurations may be employed.Additionally, the radius and lengths of the two riser sections may bevaried in order to achieve the desired results depending on catalysttype and feedstock, as described hereinabove. All such obviousmodifications are within the full-intended scope of the appended claims.

[0040] All of the above-referenced patents are hereby incorporated byreference.

We claim:
 1. An apparatus for the catalytic cracking ofhydrocarbonaceous feedstocks comprising: (a) a first narrower riserreactor section having a radius x, a means for feeding a hydrocarbonfeedstock and a means for feeding cracking catalyst located in a lowerportion thereof; (b) a second wider riser reactor section having aradius y wherein the ratio of y:x ranges from about 1.1:1 to about 5.0:1operatively connected to said first narrower riser reactor section by afirst diameter transition section; (c) a riser product conduit having aninlet operatively connected to said second wider riser reactor sectionby a second diameter transition section and having an outlet operativelyconnected to a means for separating catalyst from cracked product; and(d) a disengager vessel having an upper dilute phase, and lower densephase, said upper dilute phase suitable for receiving cracked productgases and for supporting said separator means; and said lower densephase suitable for receiving catalyst from said separator means; saiddisengager vessel further comprising an outlet for removing separatedcracked gases from said separator means.
 2. An apparatus as defined inclaim 1 wherein the ratio of y:x ranges from about 1.25:1 to about2.5:1.
 3. An apparatus as defined in claim 1 wherein said first diametertransition section operatively connects said first narrower reactorsection to said second wider reactor section at an angle ranging fromabout 5° to about 30°.
 4. An apparatus as defined in claim 3 whereinsaid angle ranges from about 8° to about 20°.
 5. An apparatus as definedin claim 1 wherein said riser product conduit has a radius ofapproximately x.
 6. An apparatus as defined in claim 5 wherein saidsecond diameter transition section operatively connects said riserproduct conduit to said second wider reactor section at an angle rangingfrom about 5° to about 30°.
 7. An apparatus as defined in claim 1wherein said riser product conduit further comprises a quench injectionmeans.
 8. An apparatus as defined in claim 1 wherein said separatormeans comprises a cyclone separator.
 9. An apparatus as defined in claim1 wherein said lower dense phase of said disengager vessel is equippedwith a means for stripping hydrocarbons from the catalyst particles. 10.An apparatus as defined in claim 1 further comprising a regeneratorvessel comprising a means for receiving spent catalyst from said densephase catalyst bed of said disengager vessel, means for regeneratingsaid catalyst, and means for recycling regenerated catalyst to saidfirst narrower reactor section.
 11. A process for the fluid catalyticcracking of hydrocarbonaceous feedstocks comprising: (a) cracking saidhydrocarbonaceous feedstock in the presence of a cracking catalyst in afirst reaction zone at a temperature ranging from about 925° F. to about1350° F. and a weight hourly space velocity greater than about 50 hr⁻¹to produce an intermediate cracked product rich in gasoline; (b)cracking said intermediate cracked product rich in gasoline in thepresence of said catalyst in a second reaction zone at a temperatureranging from about 900° F. to about 1250° F. and a weight hourly spacevelocity ranging of less than about 30 hr⁻¹ to produce a cracked productrich in propene and butenes and spent catalyst; (c) separating saidspent catalyst from said cracked product rich in propene and butenes.12. A process as defined in claim 11 wherein the temperature in saidfirst cracking reaction ranges from about 1000° F. to about 1150° F. 13.A process as defined in claim 11 wherein said weight hourly spacevelocity in said first cracking reaction ranges from about 50 to about200 hr⁻¹.
 14. A process as defined in claim 13 wherein said weighthourly space velocity in said first cracking reaction ranges from about70 to about 80 hr⁻¹.
 15. A process as defined in claim 11 wherein theconversion in said first cracking reaction ranges from about 35 to about60 percent.
 16. A process as defined in claim 11 wherein dilution steamin an amount up to about 20 weight percent based on the weight of saidhydrocarbonaceous feedstock is added to said first reaction step.
 17. Aprocess as defined in claim 11 wherein the temperature in said secondreaction step ranges from about 900° F. to about 1250° F.
 18. A processas defined in claim 11 wherein the weight hourly space velocity in saidsecond reaction step ranges from about 5 to about 20 hr⁻¹.
 19. A processas defined in claim 11 wherein dilution steam in an amount up to about20 weight percent based on the weight of said hydrocarbonaceousfeedstock is added to said second reaction step.
 20. A process asdefined in claim 11 further comprising quenching the separated crackedproduct stream.
 21. A process as defined in claim 11 further comprisingstripping the separated spent catalyst to removed entrained productvapors, and regenerating said stripped spent catalyst for recycling tosaid first cracking reaction.
 22. A method for converting a fluidcatalytic cracking system to an improved deep catalytic cracking systemcomprising the steps of: (a) removing a middle section of a riserreaction in the fluid catalytic cracking system to produce a lower firstnarrower riser reactor section having a radius x, a means for feeding ahydrocarbon feedstock and a means for feeding cracking catalyst locatedin a lower portion thereof, and an upper riser product conduit havingconnection to a cracked product/spent catalyst separation means; (b)replacing said removed middle riser section with a second wider riserreactor section having a radius y wherein the ratio of y:x ranges fromabout 1.1:1 to about 5.0:1 and operatively connecting the bottom of saidsecond wider riser reaction section to the top of said first narrowerriser reactor section by a transition reaction section and operativelyconnecting the top of said second wider riser section to the bottom ofsaid upper riser product conduit.